The conventional swing-type actuator has such a structure as illustrated in the plan of FIG. 1(a) and the front view of FIG. 1(b). In FIG. 1, numeral 1 denotes a yoke, numeral 2 a permanent magnet, numeral 3 a strut, numeral 4 a magnetic gap, numeral 5 a moving coil, numeral 6 a junction member, numeral 7 an arm, and numeral 8 a shaft. In the following figures, like parts or portions are denoted by like numerals. The junction member 6 joins the moving coil 5, which is an air-core coil, to the arm 7.
As illustrated in FIG. 1, the swing-type actuator comprises the arm 7 formed of die-cast aluminum, a magnesium alloy or the like and the moving coil 5. The moving coil 5 is fixed to the arm 7 with an adhesive or by insert molding using a thermoplastic resin. The moving coil 5 is positioned in the magnetic gap 4 sandwiched in between the permanent magnets 2, and, when the moving coil 5 is electrified, a driving force around the shaft 8 acts on the moving coil 5, so that the arm 7 swings around the shaft 8. At that time, the smaller the gap between the moving coil 5 and the permanent magnet 2 is, the more effectively a high driving force can be obtained.
The conventional moving coil 5 is generally an air-core coil produced by providing a bond wire comprising a coated wire having an adhesive film on the surface thereof, spirally winding the bond wire into a coil while applying heat or an alcohol thereto to fuse the adhesive film and cooling the wound wire. However, in the air-core coil, the dispersion of diameter size of the above bond wire is so large that constant adjustment is required in the winding step, thereby permitting only an individual winding. Further, complete in-parallel winding is difficult, so that it is hard to render the end faces of the coil parallel with each other and, therefore, the dispersion of coil density is large. Therefore, there is raised a problem that it is not feasible to adjust the gap between the moving coil 5 and the permanent magnet 2 to a desired level whereby an effective driving force can be obtained.
Meanwhile, in Japanese Patent Appln. Laid-Open Gazette No. 99756/88 and U.S. Pat. Nos. 5,122,703, 5,165,090, 5,168,185 and 5,168,184, it is proposed to join an arm 7 to a moving coil 5 which is an air-core coil with use of a holding member according to the insert molding technique. In the above proposals, there is raised a problem that the air-core coil is deformed by the pressure under which the thermoplastic resin is charged, thereby causing maintaining the parallel relationship to be unattainable. In addition, there is raised another problem that, because of the use of the air-core coil, automatic incorporation of the coil into a metal mold in the insert molding is difficult. inevitably resulting in extremely poor productivity.
Further, when the air-core coil is attempted to be used, the positions of leads where the winding is started and where the winding is ended are indefinite, so that the electrical connection of leads to terminal pins is performed by manual operations including lead positioning and removal of its coating, binding to the terminals and soldering.
Still further. when the moving coil 5, arm 7 and terminals are positioned and joined together by bonding or integral molding, there is accompanied with a difficult manual work of burying the terminals into apertures with a pair of tweezers. In the integral molding, there are frequently by caused troubles such as floating of lead, short circuit and breaking of wire. The movable coil is an air-core coil, and, for increasing the mechanical strength of the movable coil and positioning thereof in the assembly step, two methods would be contemplated, one comprising pressing a bobbin into the air-core coil after the fitting-up of the air-core coil to effect a bond therebetween and the other comprising also effecting a bond between the coil and the bobbin within the coil with a thermoplastic resin concurrently with the integral molding of the coil and the arm. In the former method, it is likely that the bobbin flaws the coil at the time of pressing the bobbin and that the coil suffers its corrosion or a change in weight balance by the evaporation of gases from the adhesive. On the other hand, in the latter method, there is raised a problem that, after the formation of the bobbin with the resin, shrinkage and other changes with the elapse of the time occur inside to thereby weaken the adhesion of the bobbin to the coil.
FIG. 2 is a perspective view of a mechanism for positioning a magnetic head for use in the conventional magnetic disk unit. In this figure, numeral 9 denotes the mechanism for positioning a magnetic head, and numerals 10, 11, 12 and 13 denote a magnetic circuit, a supporting plate, a magnetic head and a pin for positioning, respectively.
Referring to the figure, the arm 7 of the magnetic head positioning mechanism 9 serves to hold the magnetic head 12, and the rotation shaft 8 functions as the center of rotation of the arm 7 and is fixed by the supporting plate 11. The moving coil 5 is secured to the arm 7, and the magnetic flux generated by the electrification of, or application of a current to, the moving coil 5 passes through the magnetic circuit 10. The positioning of the arm 7 is performed by the positioning pin 13.
Referring further to FIG. 2, the action of the conventional actuator will be described. Upon electrification of the coil 5 secured to the arm 7 and disposed in the magnetic circuit 10, the thus generated magnetic flux passes through the magnetic circuit 10 to induce a driving force according to the Fleming's left-hand rule thereby to rotate the arm 7 holding the magnetic head 12 about the rotation shaft 8. This rotation shaft 8 is supported by a supporting plate 11 and is pressed in or bonded to a bearing not shown.
The conventional actuator for a disk unit has the above structure, so that it is necessary to decrease the weight of the whole of the movable parts in order to decrease the moment of inertia of the movable parts which determines the level of the driving force. Accordingly, measures have been taken, such as changing the material of the above arm from aluminum (specific gravity: 2.7) to magnesium (specific gravity: 1.8). There is raised, however, a problem that there is a limit in the above decrease of the weight to thereby cause a desirable miniaturization of the driving parts to be unfeasible.
For resolving the above problem, the above U.S. patents propose to effect integral molding of the arm and the holding member for holding the moving coil with a thermoplastic resin having a longitudinal elastic modulus of at least 30.times.10.sup.4 kg/cm.sup.2. However, unfavorably, the bearing part is made of the resin, so that the strength thereof is low to thereby lower the reliability of the resultant actuator. Further, since the holding member holds the peripheral part of the moving coil consisting of a multilayer air-core coil, the injection pressure deforms the air-core coil thereby to unfavorably lower the dimensional accuracy thereof.